Automotive vehicles and small vessels use D.C. electrical power sources for operation of lights and controls; the traditional power source for these applications once was a D.C. generator driven from the vehicle engine. More recently, with major improvements in rectifier technology, the D.C. generator has been replaced by the combination of a small alternator and a rectifier. The most practical and most widely used type of alternator employs a rotating magnetic field, using a field coil mounted in a core formed by two magnetic steel core members with interleaved finger-like pole pieces. For these magnetic core members, precision manufacture is essential.
Processes that have been employed in the manufacture of magnetic rotor core members for alternators and like dynamoelectric machines include cold forging (or cold extrusion) processes, cold forming stamping processes, hot forging processes, and combinations of hot forging, cold forging, and machining processes. These manufacturing procedures have each incorporated methods and techniques that have been developed independently and separately for each. Though significant improvements and advances in all of these methods have been achieved during past years, each of the known processes nevertheless still presents drawbacks and disadvantages which have proved difficult or impossible to overcome. Accordingly, each of these methods still leaves much to be desired in terms of yield rate, productivity, equipment required, etc.
For instance, the cold forging or cold extrusion method requires a large scale, high capacity press that affords an extremely high processing force. This presents substantial problems with respect to operating life and productivity of the tooling employed in the press. The cold forming stamping process presents a distinct disadvantage with respect to excessive consumption of the material from which a preliminary core blank is punched and an undesirable low yield rate. Further, this process cannot create an integral hub section, as used in many rotor core members, so that a separate rotor core spacer or hub has to be manufactured by some other process.
The hot forging process is inherently a higher yield rate procedure that has the further advantage of requiring less processing force than cold forging. However, hot forging alone is inadequate in attaining high dimensional accuracy and also is poorly adapted to producing a shaft aperture in the hub of the rotor core member. Consequently, the basic hot forging process must be followed by a number of machining steps to achieve the required finished form with precision controlled dimensional tolerances.
The best previously known methods of manufacturing magnetic rotor core members for dynamoelectric machines are described in the inventor's earlier U.S. Pat. No. 4,558,511 issued Dec. 17, 1985 and U.S. Pat. No. 4,759,117 issued July 26, 1988. Each employs a combination of hot forging and cold forging operations, and each has some operations like the present invention. Thus, each patented process, and the present invention, may employ the steps of cutting a segment from a steel bar, hot forging that segment to form a preliminary core blank with pole piece fingers, de-burring the preliminary core blank, gradually air cooling the blank, and cold compressing the blank. The prior patents also include a coining step.
According to the inventor's U.S. Pat. No. 4,558,511, the air-cooled blank may be directly cold-compressed; alternatively, rough machining may be needed. A problem with this process is that conventional hot-forged core blanks still include some excess volume even after being de-burred. These core blanks, more often than not, can be "die busters" when inserted directly into cold-compression dies. Thus, the alternative rough machining is most often necessary. At the same time, some parts of the core blank may be incomplete due to defects resulting from failure of the steel to flow into all parts of the die set during initial forging; this is a particular problem with the pole piece fingers. Rough machining is also used in the process of the inventor's later U.S. Pat. No. 4,759,117. In that process, the pole piece fingers start out shorter than required for the final rotor core, and are subsequently lengthened and ironed to final shape.